A conventional TV set has a screen with a standard aspect ratio of 4:3, whereas a TV set in recent years or in the next generation, known as an HDTV (High-Definition TV) or EDTV (Extended-Definition TV), adopts a wider screen with an aspect ratio of 16:9. Conventionally, an image based on a video signal with an aspect ratio of 4:3 is displayed on a screen with an aspect ratio of 16:9 by one of the following methods:
(1) the video signal is extended in a lateral direction (in width);
(2) some horizontal and vertical periodic pulses are skipped and delayed as is disclosed in Japanese Laid-Open Patent Application No. 276468/1993 (Tokukaihei No. 5-276468); and
(3) the video signal and a clock signal are controlled in a complex manner, so that a predetermined graduation level is written into a margin on the screen where an image based on the video signal with an aspect ratio of 4:3 is not displayed during a horizontal blanking period by accelerating a cycle period of the clock signal as is disclosed in Japanese Laid-Open Patent Application No. 131182/1991 (Tokukaihei No. 3-131182).
However, when a matrix type display device, such as a liquid crystal display device, displays an image with an aspect ratio of 4:3 on its screen with a wider aspect ratio of 16:9 by any of the above methods, each causes a problem.
To be more specific, in the first method, a displayed image is unnatural, because the image is extended in width alone or the entire image is not displayed as the video signal for the top and bottom portions is omitted.
In the second method, the video signal must be controlled in a complex manner to skip one in every certain number of row and column lines of the video signal and delay horizontal and vertical periodic pulses.
Also in the third method, the video signal and clock signal must be controlled in a complex manner. Further, the third method demands high-performance column drivers which can operate at the timing of a clock signal with an accelerated cycle period.